List slicing with dynamic index on [:index]

Question

I need to slice a list using negative dynamic indexes ([:-index]). This was easy until I realized that if the value of my dynamic index was 0, no items were returned, instea

Answer 1

You can use None rather than 0 to get the full slice:

>>> arr = [1, 2, 3]
>>> index = 1
>>> arr[:-index if index else None]
[1, 2]
>>> index = 0
>>> arr[:-index if index else None]
[1, 2, 3]

---

My testing:

import timeit

def jonrsharpe(seq, index):
    return seq[:-index if index else None]

def Cyber(seq, index):
    return seq[:len(arr) - index]

def shashank(seq, index):
    return seq[:-index or None]

if __name__ == '__main__':
    funcs = ('jonrsharpe', 'Cyber', 'shashank')
    arr = range(1000)
    setup = 'from __main__ import arr, {}'.format(', '.join(funcs))
    for func in funcs:
        print func
        for x in (0, 10, 100, 1000):
            print x,
            print timeit.timeit('{}(arr, {})'.format(func, x), setup=setup)

and results:

jonrsharpe
0 2.9769377505
10 3.10071766781
100 2.83629358793
1000 0.252808797871
Cyber
0 3.11828875501
10 3.10177615276
100 2.82515282642
1000 0.283648679403
shashank
0 2.99515364824
10 3.11204965989
100 2.85491723351
1000 0.201558213116